Voltage controlled oscillators of various kinds are known in the art. In general, such devices operate to provide a signal having a frequency that depends, at least in part, on an input control voltage. Such devices have many applications. For instance, in an AM stereo radio receiver, voltage controlled oscillators provide station tuning and average IF frequency for signal recovery functions.
Many applications require a voltage controlled oscillator having an accurate output over a relatively wide tuning range. One prior art approach intended to meet this need provides a resonant circuit, such as an inductor-capacitor pair, that connects to both a phase shifting network and a steering network. The phase shifting network operates to phase shift 135 degrees a fed back oscillator signal (subsequent signal processing in the feedback loop results in a total effective phase shift of 90 degrees (for a balanced condition) to counterbalance a 90 degree phase shift that inherently occurs in the resonant circuit itself. The steering network receives an unshifted signal from an input buffer. A summing junction then sums the phase shifted signal with the output of the steering network and provides the resultant signal to the resonant circuit.
The tuning range of the above described approach will be limited by the tuning capabilities of the steering network, which in turn can effectuate no more than 90 degrees of phase control. For many applications, such a limited tuning range may be unacceptable. Further, this approach gives rise to accuracy problems even when functioning within the indicated tuning range. More particularly, as noted above, the phase shift provided by the phase shifting network approximates 135 degrees or, stated another way, 180 degrees minus 45 degrees. This typical prior art approach to phase shifting, however, requires phase shifting at a sensitive point on the frequency versus phase curve, since the slope of this curve is maximum at 45 degree positions (i.e., 45 degrees, 135 degrees, 225 degrees, etc.). Because of this, any tolerance errors in the devices utilized to accomplish the phase shift, or any errors that result do to temperature sensitivity, can give rise to significant phase errors that can in turn cause inaccurate frequency control of the voltage controlled oscillator.
There therefore exists a need for a voltage controlled oscillator capable of providing accurate frequency control over a relatively wide tuning range.